The use of network deployment at the same frequency in a Long Term Evolution-Advanced (LTE-A) system has significantly improved the utilization ratio of spectrums but may cause a signal of a user at the edge of a cell to be seriously faded and also subjected to high interference from another cell, and an experience of the user at the edge may be seriously degraded if the issues of signal fading and interference fail to be handled.
With the technology of Coordinated Multi-Point transmission/reception (CoMP), information exchanges and joint transmission between multiple cells have been introduced so that the quality of the signal can be improved and also inter-cell interference can be lowered to thereby significantly improve the performance of data transmission for the user at the edge of the cell.
Existing general coordinated multi-point transmission schemes can fall into four categories: Dynamic Point Selection (DPS), Dynamic Point Blanking (DPB), Coordinated Scheduling/Beam-forming (CS/CB) and Joint Transmission (JT). In a practical application, there can be a hybrid scheme of the four transmission schemes. For example, DPS and CS/CB are combined so that a primary transmission point is selected dynamically and cooperating transmission points other than the primary transmission point are scheduled jointly for coordinated beam-forming, etc. In a real system, there may be multiple transmission schemes in a transmission mode to support dynamic switching between the various transmission schemes. In order to better guarantee the performance of transmission and the stability of the system, switching to transmission by a single point tends to be supported in a transmission mode supporting CoMP transmission.
Different Channel State Information-Reference Signals (CSI-RSs) or Cell-specific Reference Signals (CRSs) are configured in the system so that a User Equipment (UE) (or referred to as a user terminal) can measure downlink channels of the respective transmission points. Notably the transmission points each may not be a physical transmission point but may be a virtual transmission point, where each virtual transmission point corresponds to a CSI-RS resource, and each virtual transmission point is consisted of one or more physical transmission points.
Information are calculated according to Channel State Information (CSI) required for the transmission schemes and fed back after the downlink channels are measured. The various transmission schemes need to be supported by the corresponding channel state information. For example, the channel state information of the multiple transmission points or the channel state information of some transmission point and indication information corresponding to the transmission point needs to be fed back in the DPS transmission scheme, channel quality information under respective interference assumptions needs to be fed back when DPB is combined with DPS transmission scheme, and the CSI with the same rank needs to be fed back by the multiple transmission points in the JT transmission scheme, etc. A measurement set refers to a set of transmission points for the channel state information to be measured by the UE, or to a set of reference signal resources, each of which represents one or more transmission points, and then taking the measurement set including two Transmission Points (TPs) as an example, the channel state information required for the respective transmission schemes is as depicted in Table 1:
TABLE 1Channel state informationTransmitted signalInterferenceassumptionassumptionTP1TP2TP1TP2CQI1RI1/PMI1—OffOnCQI2,RI2/PMI2—OffOffCQI3—RI3/PMI3OnOffCQI4—RI4/PMI4OffOffCQI5RI5/PMI5RI5/PMI6OffOff
Where the CQI1 (the CQI stands for Channel Quality Indicator) is calculated assuming that a signal is transmitted from the TP1, and a Pre-coding Matrix Indicator (PMI) and a Rank Indicator (RI) of the signal transmission are the RI1/PMI1 calculated from a channel from the TP1 to the UE, and the signal may be subjected to interference from a signal transmitted by the TP2 to another user;
The CQI2 is calculated assuming that a signal is transmitted from the TP1, and a PMI and an RI of the transmitted signal are the RI2/PMI2 calculated from the channel from the TP1 to the UE, and no user will be scheduled by the TP2 on the corresponding resource, so the signal will not be subjected to interference from the TP2;
The CQI3 is calculated assuming that a signal is transmitted from the TP2, and a PMI and an RI of the transmitted signal are the RI3/PMI3 calculated from a channel from the TP2 to the UE, and the signal may be subjected to interference from a signal transmitted by the TP1 to another user;
The CQI4 is calculated assuming that a signal is transmitted from the TP2, and a PMI and an RI of the transmitted signal are the RI4/PMI4 calculated from the channel from the TP2 to the UE, and no user will be scheduled by the TP1 on the corresponding resource, so the signal will not be subjected to interference from the TP1;
The CQI5 is calculated assuming that signals are transmitted from both the TP1 and the TP2, and PMIs and RIs of the transmitted signal of the TP1 are the RI5/PMI5 and the RI6/PMI6 calculated from the channel from the TP2 to the UE, and both the TP1 and the TP2 transmit data to the UE, so neither of them will cause interference.
Both the CQI1/RI2/PMI1 and the CQI2/RI2/PMI2 are calculated assuming that the signals are transmitted from the TP1 except that there is interference or no interference to the UE from the TP2, which correspond to two different interference assumptions. In this case, it can be assumed that RI1=RI2 and PMI1=PMI2 so that the UE only needs to feed back the CQI1/RI1/PMI1 and the CQI2, thus saving an overhead of feeding back the RI2/PMI2. Alike it can be assumed that RI3=RI4 and PMI3=PMI4, thus saving an overhead of feeding back the RI4/PMI4. Furthermore it can be assumed that RI5=RI1; PMI5=PMI1; RI6=RI3; and PMI6=PMI3. Moreover the CQI5 is for the purpose of supporting JT transmission where two transmission points are required to transmit the same number of data layers, i.e., RI5=RI6. In order to perform the function above, it is necessary to introduce the interdependency between the reported various channel information of the respective transmission points.
In order to support dynamic switching between the various CoMP transmission schemes, the UE needs to feed back channel information of one or more transmission points including one or more CQUPMURI values under different interference assumptions. There are two existing practices to feed back the CSI as required for the CoMP schemes:
Firstly a new feedback mode (i.e., report mode) is designed so that all the information is arranged in the same mode to be reported, that is, a report periodicity and a sub-frame offset are configured uniformly, and all the information is reported sequentially to a base station in some order. This solution is advantageous in the easy introduction of the interdependency between the various reported information but inflexible in that the feedback mode may be changed due to any of a change to the number of transmission points in the measurement set, a change to the feedback information and a change to the preset interdependency, that is, the feedback mode thereof has to be designed taking the various possible situations into account.
Secondly the information to be reported is distributed into multiple feedback modes, each of which is configured separately with a report periodicity, a sub-frame offset and other parameters. A problem of this solution lies in that is not easy to introduce the interdependency between the reported information, particularly when the interdependent information is distributed into the different report modes.